To the end of improving the responsiveness and stability of a vehicle in a turning maneuver, various forms of front and rear wheel steering systems have been proposed. Normally, the rear wheels are steered according to the steering angle of the front wheels effected by the turning of the steering wheel, and various parameters of the vehicle such as vehicle speed are used for determining the steering angle of the rear wheels or the ratio of the rear wheel steering angle to the front wheel steering angle.
It has been established that a front and rear wheel steering system such as a four wheel steering system is highly effective in improving the behavior of the vehicle when the steering wheel is turned by the will of the vehicle operator, for instance, for changing lanes and avoiding an object. For details of conventional front and rear wheel steering systems, reference should be made, for instance, to U.S. Pat. No. 4,313,514 issued Feb. 2, 1982 to Furukawa et al.
However, if the rear wheels are simply steered only in association with the steering angle of the front wheels, it is not possible to effectively control the behavior of the vehicle when subjected to external disturbances such as side wind or cross wind and irregularities of the road surface.
Therefore, recently, there have been attempts to include the dynamic behavior of the vehicle in the parameters for determining the steering angle of the rear wheels. In particular, it has been proposed to detect the yaw rate of the vehicle, and use it as a parameter for determining the steering angle of the rear wheels. More specifically, when a yaw rate is detected and this yaw rate is determined to be caused not by the turning of the steering wheel, it is assumed that the vehicle is subjected to external disturbances such as side wind, and the rear wheels are automatically steered so as to cancel this yaw rate.
It is generally believed that the driveability of a vehicle is more improved as the delay in the yaw rate response is smaller, and it is known that the yaw rate response can be improved if the front and rear wheels are steered in an opposite phase relationship in an early phase of turning the steering wheel. However, when this is actually implemented, the vehicle operator may experience an unfamiliar impression because when the vehicle operator turns the steering wheel in one direction the rear wheels are initially steered in an opposite direction and the rear part of the vehicle is slightly swung in an unexpected direction.
When the vehicle is turned in a downwind direction by side wind, and an effort is made to turn the vehicle toward the wind by steering the rear wheels in an attempt to restore the vehicle to the straight ahead direction, the rear part of the vehicle has to be swung in downwind direction, and the vehicle has to make a highly distinct lateral shifting movement to keep the vehicle on a straight course.
As an additional problem, the vehicle normally involves a certain side slip angle, and the resulting slight discrepency between the direction of the actual movement of the vehicle and the orientation of the vehicle may impair the driveability of the vehicle. Normally, a vehicle tends to be oriented outward with respect to the tangential line of the turning radius of the vehicle in a low speed range. Conversely, in a high speed range, a vehicle tends to be oriented inward with respect to the tangential line of the turning radius of the vehicle.